The present invention relates to a ladder-type electric filter device to be used for a filter circuit of a portable and mobile radio communication unit such as an automobile telephone set, in which it is mounted on a print-circuit board to form a desired filter circuit and comprises series and parallel resonators contained in a case as components of a unit filter circuit.
A conventional ladder-type electric filter device normally comprises series resonators one of whose terminal electrodes constitutes an input side, and parallel resonators one of whose terminal electrodes is grounded, the other terminal electrodes of the series and parallel resonators constituting an output side.
As a variety of such a ladder-type electric filter, one comprising square series and parallel resonators having a side approximately 5 mm long and utilizing a peripheral oscillation mode, and a filter comprising circular series and parallel resonators having a diameter of approximately 5 mm and utilizing a radial oscillation mode and designed to operate with a center frequency of 455 KHz are already known.
Recently, in an attempt of providing down-sized and lightweight radio communication units, there has been an increasing demand for thin ladder-type electric filters to be used for filter circuits. However, with a conventional filter comprising resonators which are as wide as approximately 5 mm, the overall height of the casing that contains them needs to be inevitably rather large and does not allow any further reduction.
Further, there has been proposed rectangular resonators utilizing a longitudinal oscillation mode to be used in place of square or circular series and parallel resonators. While the use of rectangular series and parallel resonators can reduce the height of the filter, it entails the necessity of preparing small rectangular input, output, grounding and connection terminal plates for holding the respective resonators. However, since the terminal plates have an extremely small thickness of less than 0.1 mm, the operation of handling such thin plates to fit them into a case is not an easy task, making the overall manufacturing cost of a filter rather high.